1. Field of the Invention
This invention relates to a method of and a system for controlling injection timing in a diesel engine, and more particularly to improvements in a method of and a system for controlling injection timing in a diesel engine, suitable for use in an electronically controlled motor vehicle diesel engine having an ignition timing sensor, wherein the injection timing is feedback-controlled in accordance with a deviation between a target ignition timing, determined on the basis of an engine load and an engine speed, and an actual ignition timing, detected by an ignition timing sensor.
2. Description of the Prior Art
Japanese Patent Laid-Open Nos. 28842/1982, 192935/1983, 153942/1984 and U.S. Pat. No. 4,463,729 disclose that when controlling injection timing to optimize the exhaust gas purifying performance and the like of a diesel engine, particularly for diesel engine for a motor vehicle, the use of an ignition timing sensor, such as a flame sensor installed in a combustion chamber, results in the detection of ignition timing, that is, a timing at which pressure in a cylinder rapidly rises by combustion, or a timing at which a light of combustion by ignition rises. The ignition timing is detected in a combustion chamber by the ignition timing sensor and is fed back so that the actual ignition timing becomes a required ignition timing as determined by an engine load, an engine speed and the like. By feedback-controlling the injection timing in response to an output from the ignition timing sensor as described above, a wave form signal from the ignition timing sensor is shaped by a predetermined threshold value Vth and an actual ignition timing is calculated from a difference between the ignition signal and a reference position, such as top dead center (TDC).
When the ignition timing sensor malfunctions due to a disconnection or the like and no ignition signal is outputted, the applicant has proposed in Japanese patent application Nos. 205248/1983 and 82386/1983 that by monitoring the presence of an ignition signal outputted from the ignition timing sensor and detecting the absence of an ignition signal within a preset time period, an open loop control is effected in response to an injection start timing signal, as disclosed in Japanese patent application No. 205248/1983, and a fuel injection timing set on the basis of an engine speed and an engine load, as disclosed in Japanese patent application No. 82386/1983.
However, since the ignition timing sensor has its forward end face exposed to an engine combustion chamber, during operating conditions at low temperature or at low speed and high load, carbon may adhere to the glass front end face at the forward end of the ignition timing sensor. In that case, an output from the ignition timing sensor is lowered considerably, and, in the extreme case, no ignition signal may be outputted.
As a result, in order to accurately judge the presence of an ignition signal, it is necessary to judge the presence of an ignition signal in a specific region wherein the ignition signal is stable. Therefore, since there is a possibility that the ignition timing sensor smolders during the idling of the engine, at a time of low engine rotation or at a time of low engine load, the presence of an ignition signal and trouble in the ignition timing sensor cannot be detected. Therefore, injection timing is controlled by a maximum spark advance or a maximum spark lag, thus presenting such disadvantages as increased diesel knocking and increased white smoke.
Furthermore, if the presence of a malfunction is mistakenly judged during conditions in which the ignition signal is instable, fail safe controls such as trouble diagnosis, trouble display and decreased fuel quantity are effected. In smoldering conditions in which carbon adheres, it becomes difficult to effect the feedback control in response to an ignition signal. However, it is possible to effect control of the ignition timing by the open loop control in response to a basic injection timing determined from an engine speed, an acceleration opening and the like, whereby there is no need to effect fail safe controls as is necessary when the ignition timing sensor malfunctions. Furthermore, since the smoldering can be overcome by high engine rotation, there is no need to annoy a vehicle driver with a trouble display.